This invention relates to a centrifugal fluid pump assembly for pumping a medical fluid, typically blood.
In modem medical treatment, centrifugal blood pumps are often used in artificial heart/lung units for extracorporeal blood circulation. Centrifugal pumps of the magnetic coupling type wherein a driving torque from an external motor is transmitted to the impeller through magnetic coupling are commonly used because the physical communication between the blood chamber of the pump and the exterior can be completely excluded to prevent invasion of bacteria.
In general, centrifugal blood pumps include a housing having a blood inlet port and a blood outlet port and an impeller accommodated for rotation in the housing for feeding blood by a centrifugal force developed during rotation. The impeller having magnetic pieces of permanent magnet disposed therein is rotated by a rotational torque generating mechanism which includes a rotor having magnets for attracting the magnetic pieces of the impeller and a motor for rotating the rotor. The magnetic pieces of permanent magnet in the impeller are generally located within a shroud of the impeller as shown in FIG. 3 of Japanese Pat. Publication (JP-B) No. 23114/1982 and U. S. Pat. 5,112,202. JP-B 23114/1982 discloses in FIG. 7 another embodiment wherein magnetic pieces are located in a magnet chamber separately formed outside the blood chamber.
In the latter embodiment wherein magnets are received in the magnet chamber as shown in FIG. 7 of JP-B 23114/1982, a seal is provided between the blood and magnet chambers for preventing blood from penetrating into the magnet chamber. If a negative pressure develops within the pump as a result of accidental blockage of a portion of the circuit upstream of the pump, the direction of the pressure load applied to the seal is reversed, increasing the risk that air in the magnet chamber be sucked into the blood chamber.
The centrifugal pumps wherein magnets are located in the impeller shroud are substantially free of such a risk because the volume of air in the bearing chamber is very small. However, the height of the blood pump chamber is increased by the height of magnets to increase the amount of blood with which the chamber is filled. It is also known for centrifugal pumps that a radial imbalance of pressure distribution creates a radial thrust which is in proportion to the radially projected area of the impeller. Therefore, centrifugal pumps of this type have the problem that an increase of the axial height of the impeller leads to an increase of the radially projected area of the impeller which in turn, increases the radial thrust developed by a radial imbalance of pressure distribution, which in turn, increases the radial load on the bearing.